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"Such findings present a situation that is particularly worrisome given that infants and children are especially vulnerable to the effects of lead poisoning," said Dr Tsanangurayi Tongesayi, from Monmouth University in New Jersey, who led the study.

The findings were presented at the annual meeting of the American Chemical Society in New Orleans.

The researchers are still in the process of analysing rice samples from Pakistan, Brazil and other countries.

Lead accumulates slowly in the body, and can lead to nerve and kidney damage, as well as anaemia.

One study has shown brain shrinkage in workers exposed to lead through their occupations.

Long-term lead exposure has been linked to reduced IQ and disruptive behaviour in children.

Rice is the staple food of around three billion people worldwide.

In the UK, the average person consumes around 5.6 kg of rice per year. Consumption in the UK is expected to increase as the ethnic population expands and food tastes diversify.

Rice imports account for only about 7 percent of the rice consumed in the US, which is a major producer and exporter of the grain.